The invention pertains to machine vision and, more particularly, to methods and apparatus for illuminating objects under inspection. The invention has application in machine vision inspection of stationary and moving objects, such as moving webs.
Machine vision refers to the automated analysis of images to determine characteristics of objects shown in them. One of its applications is in web manufacture, where continuous sheets are inspected for defects as they move from station to station or as they are rolled for shipping. For example, in the manufacture of sheet metal, images are analyzed to insure that the sheets are not scratched or otherwise defective.
Machine vision has wide application in other industries as well. It can be used in the semiconductor device industry, for example, to analyze images of boards, chips and other components to verify their integrity. It can also be used in the paper industry to detect blemishes on or near the surface of the paper.
The prior art suggests the use of "dark field" imaging to facilitate the detection on the surfaces of objects of features (or defects) that cast shadows such a scratches, rips, dents or lumps. According to this technique, an imaging device (e.g., camera) is placed outside the usual pathway along which light from the illuminating source is reflected from the object.
Dark field imaging is effective because it highlights features or defects that protrude from (or into) the surface of the object being inspected. A dark field image of a smooth reflective semiconductor surface, for example, is uniformly dark, since none of the light from the illumination source is reflected to the camera. If that surface is scratched, however, light reflecting from the edges of the scratch is reflected to the camera, thereby, revealing the imperfection.
To date, the art has had only limited success in applying dark field imaging to the inspection of moving webs. One prior art scheme calls for aiming a camera along a line normal to the surface of a web that is illuminated obliquely with diffuse light, e.g., a fluorescent tube. A problem with this arrangement is that the images typically fail to reveal all but the grossest defects in the material.
Another prior art scheme for illuminating moving webs is laser sidelighting. According to that technique, a beam of non-diffuse light, e.g., laser light, is used to illuminate the web obliquely from the side, while a camera images the web along a line normal to the surface of the web. A problem with laser sidelighting is that it brings out so much detail in the web itself that extraordinary processing power and time is required to identify the defects. Another problem is that interference between the reflected rays of laser light causes artifacts (e.g., a speckle pattern consisting of "sparkles" or dots) that further complicate analysis.
In view of the foregoing an object of this invention is to provide improved methods and apparatus for machine vision and, particularly, improved methods and apparatus for illuminating objects under inspection.
A further object of the invention is to provide such improved methods and apparatus as can be applied in dark field imaging of objects, such as moving webs, to highlight characteristics of interest, e.g., defects.
Yet another object of the invention is to provide such methods and apparatus as are suitable for highlighting characteristics of interest without unduly highlighting those that are not of interest.
Still another object of the invention is to provide such methods and apparatus as can be used with "line" cameras, i.e., cameras that generate images of linear (vs planar) fields of view, as well as conventional planar field of view cameras.
Yet still another object of the invention is to provide such methods and apparatus as can be implemented without undue cost and use of specialized equipment.